Dialkyl carbonates are important intermediates for the synthesis of fine chemicals, pharmaceuticals and plastics and are useful as synthetic lubricants, solvents, plasticizers and monomers for organic glass and various polymers, including polycarbonate, a polymer known for its wide range of uses based upon its characteristics of transparency, shock resistance and processability.
One method for the production of polycarbonate resin employs phosgene and bisphenol-A as starting materials. However, this method has numerous drawbacks, including the production of corrosive by-products and safety concerns attributable to the use of the highly toxic phosgene. As such, polycarbonate manufacturers have developed non-phosgene methods for polycarbonate production, which use diphenyl carbonate and bisphenol-A as starting materials. Diphenyl carbonate can be prepared from phenol and dimethyl carbonate.
Dimethyl carbonate has a low toxicity and can also be used to replace toxic intermediates, such as phosgene and dimethyl sulphate, in many reactions, such as the preparation of urethanes and isocyanates, the quaternization of amines and the methylation of phenol or naphthols. Moreover, it is not corrosive and it will not produce environmentally damaging by-products. Dimethyl carbonate is also a valuable commercial product finding utility as an organic solvent, an additive for fuels, and in the production of other alkyl and aryl carbonates.
Dimethyl carbonate, as well as other dialkyl carbonates, have traditionally been produced by reacting alcohols with phosgene. These methods have the same problems as methods that use phosgene and bisphenol-A, i.e., the problems of handling phosgene and disposing of phosgene waste materials. Thus, there is a need for commercially viable non-phosgene methods for the production of dimethyl carbonate, as well as other dialkyl carbonates.
Non-phosgene methods that have been proposed for producing dialkyl carbonates include the transesterification reaction of alcohols and cyclic carbonates. Most of the proposed methods relate to the use of various catalysts for that reaction. Examples of such proposed catalysts include alkali metals or basic compounds containing alkali metals; tertiary aliphatic amines; thallium compounds; tin alkoxides; alkoxides of zinc, aluminum and titanium; a mixture of a Lewis acid and a nitrogen-containing organic base; phosphine compounds; quaternary phosphonium salts; cyclic amidines; compounds of zirconium, titanium and tin; a quaternary ammonium group-containing strongly basic anion-exchange solid material; a solid catalyst selected from the group consisting of a tertiary amine- or quaternary ammonium group-containing ion-exchange resin, a strongly acidic or a weakly acidic ion-exchange resin, a mixture of an alkali metal with silica, a silicate of an alkaline earth metal and an ammonium ion-exchanged zeolite; and a homogeneous catalyst selected from the group consisting of tertiary phosphine, tertiary arsine, tertiary stibine, a divalent sulfur compound and a selenium compound.
The catalytic transesterification of a cyclic carbonate with an alcohol generally involves two equilibrium steps which typically generates a hydroxyalkyl carbonate as the reaction intermediate. For example, in the transesterification of ethylene carbonate (EC) with methanol (MeOH), the intermediate which is formed is 2-hydroxyethyl methyl carbonate (HEMC). This two equilibrium step reaction may be represented by the following: 
These reaction steps for converting the cyclic carbonate and alcohol to the dialkyl carbonate generally occur as two sequential steps. Addition of the first molecule of alcohol to the cyclic carbonate results in the production of the intermediate hydroxy alkyl carbonate. Addition of the second molecule of alcohol to the intermediate results in the production of the dialkyl carbonate and diol. The intermediate hydroxy alkyl carbonate generally builds to a maximum concentration faster than the equilibrium dialkyl carbonate concentration is reached. As a result of equilibrium constraints on the reactions, a maximum concentration (i.e., the equilibrium concentration) will be reached for the desired products. Thus, there is a limit to the yield for producing dialkyl carbonates and diols from cyclic carbonates and aliphatic monohydric alcohols for a given catalyst and reaction conditions.
Unsymmetric dialkyl carbonates, such as ethyl methyl carbonate (EMC), are useful as solvents for electrolytic solutions for lithium rechargeable batteries, solvents for resins and coating compositions, alkylating agents, or starting materials for carbamate synthesis.
Ethyl methyl carbonate, as well as other unsymmetric dialkyl carbonates, have traditionally been produced by esterification of alkyl chloroformate with alcohol under base (pyridine or amine) catalysis. Such methods have similar problems to the methods discussed above that use phosgene and bisphenol-A, i.e., highly reactive and highly toxic starting materials.
Other methods have been disclosed for the synthesis of unsymmetric dialkyl carbonates, which avoid the use of such highly toxic starting materials. One method involves an ester exchange reaction of a symmetric dialkyl carbonate with an alcohol having a different alkyl group under base catalysis. However, such a reaction typically results in a product, which includes a mixture of three dialkyl carbonates and two alcohols. For example, when a 1:1 molar ratio of DMC and EtOH is used as the starting materials, the product mixture will typically contain about a 45:45:10 molar ratio of DMC:EMC:DEC (diethyl carbonate) and a relative ratio of MeOH to EtOH of about 2:1. The mixture of these three dialkyl carbonates can result in difficult or costly purification steps to isolate the unsymmetric dialkyl carbonate, e.g., EMC.
Other methods which have been proposed include the disproportionation of two symmetrical dialkyl carbonates using a basic catalyst, e.g., an alkali metal alcoholate. However, such methods typically result in a product mixture of three dialkyl carbonates, including one unsymmetrical dialkyl carbonate. Again, this can result in difficult or costly purification steps to isolate the unsymmetric dialkyl carbonate from the three component mixture.
Thus, there is a need for a process for the production of symmetric and/or unsymmetric dialkyl carbonates and diols from starting materials which include cyclic carbonates and alcohols which does not have the above-mentioned disadvantages.